Tuesday 20 August 2013 12.01 EDT
First published on Tuesday 20 August 2013 12.01 EDT

A former undercover spy who blew the whistle on abuses of a covert Scotland Yard unit has offered to speak to an inquiry if police chiefs withdraw their threat to investigate him for breaking the Official Secrets Act.

Peter Francis, who infiltrated anti-racism campaigners for four years, has made the offer after Sir Bernard Hogan-Howe, commissioner of the Metropolitan police, and called on him to talk to the investigators.

The internal inquiry, led by the Derbyshire chief constable Mick Creedon, is scrutinising allegations surrounding the special demonstration squad, a previously secret unit that planted undercover officers in political groups for 40 years.

Francis says that since he retired from the force in 2001 he has been threatened several times by Met officers who said that if he spoke out he would prosecuted under the Official Secrets Act.

"It feels as though they are holding a legal mallet over me," he said. "That threat needs to be totally removed."

Over the past two years Francis has disclosed many details of the squad's work, including how undercover police routinely had sexual relationships with political campaigners and regularly adopted the identities of dead children to use as their aliases.

In testimony made public by the Guardian and Channel 4's Dispatches in June, Francis said the unit spied on the campaign for justice over the death of Stephen Lawrence. As part of that mission, Francis said, his superiors asked him to find information that could be used to undermine the family and their supporters. Hogan-Howe has asked Creedon to prioritise his investigation into these claims.

Francis has been reluctant to speak to Creedon's team as he has doubts that the internal police inquiry is sufficiently independent to investigate the matter, saying instead that he would give evidence under oath to a full public inquiry. He has been backed by the murdered teenager's mother, Doreen.

But Hogan-Howe has made a series of public appeals asking Francis to speak to the investigators. The latest came last week, when Britain's most senior police officer said: "One of the critical things is to talk to Peter Francis. We've seen him appear on television. We know he's talked to journalists. It would be really helpful if he talked to the investigators because that will help to get to the bottom of it."

On 26 July, two officers from Creedon's team visited Francis at his home and asked him to talk to the investigation. He told them that he would only consider speaking to the inquiry if the threat of an official secrets investigation was lifted.

Francis said that while he did not believe prosecutors would bring a case against him, as he hoped his disclosures had been in the public interest, police could still try to investigate and arrest him as punishment for speaking out.

Francis said that if the police confirmed he would not be investigated for divulging official secrets, he would then talk to Hogan-Howe or Creedon to see if they could offer assurances that the investigation would be completed properly. "I want to see if they have the stomach to do this credibly," he said.

Francis made his offer after concluding that a report published by Creedon in July on one aspect of the SDS – the practice of stealing dead children's identities – was "not a total whitewash".

Francis has for a long time favoured an independent public inquiry where he could testify on oath. Doreen Lawrence has backed Francis's stance as she and the teenager's father, Neville, see a public inquiry as the only way of getting to the truth.

Her lawyer, Imran Khan, said Francis "should be giving evidence to a public inquiry, not staying behind closed doors and speaking to police officers. That's the whole point of calling for an inquiry".

After the former spy's claims were broadcast in June, Francis wrote on 1 July to the home affairs select committee offering to give evidence in public "about any aspect of his deployment including gathering intelligence on the Stephen Lawrence campaign". The committee has been holding an inquiry into undercover policing.

Since Francis made his claims, Scotland Yard has admitted that undercover officers were deployed to spy on the supporters of the campaign by the Lawrence family for justice after the Met failed to properly investigate their son's murder.